1. Field of the Invention
The present invention relates generally to a projector, and more particularly to an automatic focusing projecting device and a method of automatic focusing.
2. Description of the Related Art
Projectors are wildly used in business, education, and home life. In present days, portable micro-projectors are provided in market. Such projectors have a small size, and some of them are incorporated in camera, video camera, or cell phone.
To project a clear image on screen, user has to manually adjust the lens to a predetermined position on the optical axis to form the image on the screen, and that is so called “focusing”. Most of the conventional projectors are equipped with manual focus system.
For a large projector, the lens is big and heavy, and the distance of the movement of the lens for focusing is long so that to manually adjust the focus of such projector is not easy for some people. However, for a micro-projector, the size of the lens is very small, and the distance of the movement of the lens for focusing is very short as well, and that is very difficult for manual focusing.
A conventional automatic focus system, taught in Taiwan patent 200412469, includes a rangefinder to find the distance between the projector and the screen, and a stepper motor to move the lens to a predetermined position. Such automatic focus system cannot identify that the image on the screen is clear or not, and the rangefinder needs large power. It cannot be used in a micro-projector.